From the above results, we have found that the FeSe11111 superconductor exhibits similar behaviors to the superconducting single-layer FeSe/SrTiO\(_3\)  film, in terms of the Fermi surface topology, band structure and the superconducting gap symmetry. Amongst all the iron-based superconductors discovered so far, the majority of them show common electronic structure with hole-like Fermi surface sheets around the \(\Gamma \)  point and electron-like Fermi surface near the M point[1]}. The A\(_x\) Fe\(_{2-y}\) Se\(_2\)  superconductor[2]}, [3]}, [4]}, [5]}, [6]}, [7]} and the superconducting single-layer FeSe/SrTiO\(_3\)  films[8]}, [9]}, [10]}, [11]} have been the only two exceptions that have distinct Fermi surface topology, i.e., no hole-like Fermi surface at the \(\Gamma \)  point and there are only electron pockets. The present work has established the FeSe11111 superconductor as a new third exception on the list. It has been proposed that the electron scattering between the hole-like bands near \(\Gamma \)  and electron-like bands near M is the cause of electron pairing in the iron-based superconductors[12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}. The discovery of only electron-like Fermi surface in the A\(_x\) Fe\(_{2-y}\) Se\(_2\)  superconductor and the superconducting single-layer FeSe/SrTiO\(_3\)  films challenges such a Fermi surface nesting scenario. However, some material-specific properties prevent us from making a decisive conclusion. The A\(_x\) Fe\(_{2-y}\) Se\(_2\)  superconductor is well-known for its phase separation problem; the superconducting phase occupies only a small fraction of the entire sample volume and the nature of the superconducting phase remains unclear[19]}, [20]}, [21]}, [22]}, [6]}, [7]}. The coexistence of the superconducting phase and the insulating 245 phase makes people wonder whether the interface between them is responsible for the observed superconductivity. On the other hand, the surprising discovery of high temperature superconductivity in the single-layer FeSe/SrTiO\(_3\)  films indicates that interface may play a significant role in inducing or enhancing superconductivity[25]}, [8]}, [9]}, [10]}, [29]}, [30]}, [31]}, [32]}, [33]}, [34]}, [35]}, [11]}. In this sense, the complications of phase separation and interface effect hinder the direct comparison of the A\(_x\) Fe\(_{2-y}\) Se\(_2\)  superconductors and superconducting single-layer FeSe/SrTiO\(_3\)  films with most other iron-based superconductors. The newly-discovered FeSe11111 superconductor is advantageous in that it is a single-phase bulk superconductor, free from complications of phase separation and interface effect, thus making it possible to compare directly with other iron-based bulk superconductors. The observation of high temperature superconductivity with a T\(_c\)  at 41 K in bulk FeSe11111 superconductor that has electron-pocket-only Fermi surface topology makes a strongest and most decisive case that the hole-like Fermi surface around \(\Gamma \)  is not necessary for the superconductivity in the iron-based superconductors. This rules out Fermi surface nesting scenario as the common pairing mechanism for the iron-based superconductors and points to the significant role of the electron pockets near M in governing high temperature superconductivity in the iron-based superconductors.
